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Using Gaussian Checkpoint Files

David Young
Cytoclonal Pharmaceutics Inc.

The Gaussian computational chemistry program allows the results of a calculation to be saved in a machine readable file, called a checkpoint file. The primary use of a checkpoint file is to use the results of one calculation as the starting point for a second calculation.

When a calculation is started using information from a checkpoint file, the calculation results will be placed in the exact same checkpoint file, overwriting the original checkpoint file. Thus it is always a good idea to make a backup copy of the checkpoint file. Note that it is possible for a checkpoint file to become corrupted (i.e. if a calculation dies while writing to the checkpoint file).

Gaussian will use a checkpoint file if the command

	%Chk=file_name

appears before the route card in the input file. If the specified file does not exist, it will be created. If the specified file does exist, information to be used in the present calculation can be read from it.

Commands for reading from the checkpoint file

A calculation can be started using information from the checkpoint file by including one of the following commands in the route card.

ChkBasis: Read the basis set from the checkpoint file.
SCF=Restart: Restart an SCF calculation from the checkpoint file. This is normally used when an SCF calculation failed finish for some reason.
IRC=Restart: Restarts an IRC calculation that did not complete, or restarts an IRC calculation for which additional points along the reaction path are desired.
Scan=Restart: Restarts a potential energy surface scan which did not complete.
Freq=Restart: Restarts a numerical frequency calculation which did not complete. Analytic frequency calculations cannot be restarted.
Polar=Restart: Restarts a numerical polarizability calculation which did not complete.
CIS=Restart: Restarts a CIS (Configuration Interaction - Single excitation) calculation which did not complete.
Opt=Restart: Restarts an geometry optimization which did not complete.
Geom=Checkpoint: Reads the molecular geometry from the checkpoint file.
Geom=AllCheckpoint: Reads the molecular geometry, charge, multiplicity and title from the checkpoint file. This is often used to start a second calculation at a different level of theory.
Guess=Read: Reads the initial guess from the checkpoint file. If the basis set specified is different from the basis set used in the job which generated the checkpoint file, then the wave function will be projected from one basis to the other. This is an efficient way to switch from one basis to another.
Density=Checkpoint: Reads the density from the checkpoint file. This inplies Guess=Only so that no integrals or SCF are computed. This is used to compute the population analysis or create cube files from a wave function without rerunning the job.
Field=Checkpoint: Reads the 34 multipole components of a finite field from the checkpoint file.
Field=EChk: Reads the 3 electric dipole field components from the checkpoint file.
Charge=Check: Reads point charges from the checkpoint file.
ReArchive: This option is used to generate an archive entry from the information in the checkpoint file. No calculation is run.

Checkpoint file Utilities

The c8694 utility can be used to convert a checkpoint file from an old format (Gaussian 86 through Gaussian 92/DFT) to the Gaussian 94 format. It is given a single argument, which is the name of the checkpoint file. The file is overwritten by a new file with the same name.

chkchk is a utility provided with the Gaussian distribution. It extracts the title and route sections from a checkpoint file. The command usage is

	chkchk file_name

where file_name.chk is the name of a checkpoint file in the current directory.

chkmove is another utility provided with Gaussian. It is used to move checkpoint files between machines of different architecture. Using ftp in binary mode to transfer a checkpoint file between machines of different architecture will not result in having a readable file on the new machine. In order to transfer a file between machines of dissimilar architecture, the steps that must be followed are

Convert the checkpoint file from an unformated file to a formated file with the command
```
		chkmove f file_name.chk file_name.xfr
```
Transfer the formatted file to the new machine using the unix rcp command or ftp in ASCII mode.
On the new machine, convert the file back into an unformated checkpoint file with the command
```
		chkmove u file_name.xfr file_name.chk
```

The cubegen utility can be used to generate cube files from a checkpoint file. This gives the same end result as using the cube keyword in the route card. cubegen is explained in the "Gaussian User's Reference".

The formchk utility can be used to generate a formated (ASCII) version of the checkpoint file. formchk is used with the command line

	formchk file_name.chk file_name.fchk

where the first file name is the binary checkpoint file and the second is the name of the file to create. Using the formchk utility after the completion of a job does not allow as much flexibility of output format as is available by using the FormCheck keyword when the job is run.

The freqchk utility can be used to extract vibrational frequency and thermochemistry information from a checkpoint file. It can also be used to create files which can be read by HyperChem and used to animate the vibrational modes.

Creating an ASCII checkpoint file

The FormCheck keyword can be used to request that an ASCII version of the checkpoint file be created. It is always given the name Test.FChk and placed in the current directory. This file is not readable by the Gaussian program. FChk and FCheck are synonyms for FormCheck. The FormCheck command has a number of options which are explained fully in the "Gaussian User's Guide".

Further Information

An introductory description of how to use Gaussian is in
J. B. Foresman, A. Frisch "Exploring Chemistry with Electronic Structure Methods: A Guide to Using Gaussian" Gaussian (1993)

Commands are described in detail in
M. J. Frisch, A. Frisch, J. B. Foresman "Gaussian 94 User's Guide" Gaussian (1996)

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